Decision Height (DH) in Aviation

Decision Height (DH) is a foundational safety concept in instrument approach procedures, ensuring pilots make timely, standardized decisions at the most critical phase of flight—landing in low-visibility conditions. This article explores the technical, operational, and regulatory facets of DH as they apply to global aviation operations.

Definition and Regulatory Context

Decision Height (DH) is defined as the specified vertical distance above ground level (AGL) on a precision approach—such as an Instrument Landing System (ILS) or Ground-Based Augmentation System (GBAS)—at which the pilot must decide whether to continue the approach and land, or execute a missed approach if the required visual references are not visible.

ICAO Annex 6:
A specified height in a precision approach or approach with vertical guidance at which a missed approach must be initiated if the required visual reference to continue the approach has not been established.

DH is measured using the aircraft’s radio altimeter, ensuring precision relative to the terrain directly beneath the approach path. This distinguishes DH from Decision Altitude (DA), which references mean sea level (MSL) and uses the barometric altimeter.

DH is explicitly published for each precision approach procedure and is a regulatory minimum, not subject to pilot discretion. Its strict observance is central to preventing Controlled Flight Into Terrain (CFIT) incidents and maintaining approach standardization across the industry.

Operational Significance

DH serves as the final go/no-go gate before landing in instrument meteorological conditions (IMC), especially where visibility or ceiling is low. When the aircraft reaches DH:

• If the required visual cues (runway threshold, lights, or markings) are visible and the aircraft is positioned for a normal landing, the approach may continue.
• If not, the pilot must immediately initiate the missed approach.

This inflexible requirement is designed to combat the human tendency to “press on” in marginal conditions, ensuring safety is prioritized over operational convenience. Regulatory authorities (ICAO, FAA, EASA) mandate strict adherence to DH to prevent accidents resulting from loss of situational awareness or misjudgment at low altitudes.

In advanced operations, such as Category III approaches, DH can be as low as 50 feet AGL or even zero (CAT IIIc), relying entirely on certified autoland systems and airport infrastructure.

DH in Instrument Approaches

Precision approaches, such as ILS, GLS, and advanced GPS-based procedures, specify a DH to ensure a safe transition from instrument to visual flight:

• Category I ILS: DH ≥ 200 ft AGL (requires basic airport lighting and aircraft equipment)
• Category II ILS: DH ≥ 100 ft AGL (requires enhanced lighting, aircraft, and crew certification)
• Category III ILS: DH as low as 50 ft AGL or none (autoland; highest certification standard)

The approach chart for each procedure specifies the DH and required visibility (Runway Visual Range, RVR). At DH, if the minimum visual references are present and the aircraft is stabilized, landing may continue. Otherwise, a missed approach is mandatory.

Required Visual References

Visual cues at DH typically include:

• Runway threshold, threshold lights, or markings
• Approach lighting system (with additional criteria)
• Touchdown zone or markings
• Runway edge or centerline lights

The exact requirements vary by regulatory authority and approach category.

Decision Height vs. Decision Altitude (DA) and Minimum Descent Altitude (MDA)

Aviation approach procedures employ several minimums to ensure safety. Understanding their differences is critical:

Key Differences:

• DH is a height (AGL) and always an immediate action point; used with precision approaches and radio altimeter.
• DA is an altitude (MSL); similar in function to DH but referenced differently.
• MDA is a “floor” for non-precision approaches; the pilot may level off at MDA until the missed approach point, not an instant action point.

Regulatory authorities prohibit descending below DH or DA without the required visual cues. At MDA, the pilot may remain level until the missed approach point.

Factors Influencing DH

Determining DH for a given approach involves consideration of multiple factors:

1. Approach Category:

   • CAT I: ≥ 200 ft AGL
   • CAT II: ≥ 100 ft AGL
   • CAT III: ≥ 50 ft AGL or none

2. Weather Conditions:

   • DH is linked to RVR; lower DH is only authorized with adequate visibility.
   • Temporary increases to DH may be published by NOTAM due to degraded conditions.

3. Airport Infrastructure:

   • Advanced approach and runway lighting systems are prerequisites for lower DH minima.
   • Maintenance and certification of ground systems are mandatory.

4. Aircraft and Crew Certification:

   • Only aircraft with certified autoland and radio altimeter systems may use the lowest DHs.
   • Crews must be trained and current in low-visibility operations.

5. Regulatory Requirements:

   • DH is published in the approach chart and enforced by aviation authorities.
   • Deviations are not permitted without specific regulatory approval.

Pilot Decision-Making at DH

At Decision Height, the pilot’s actions are governed by clear regulatory and procedural requirements:

1. Assessment:

   • Are the required visual references distinctly visible?
   • Is the aircraft in a position for a safe, normal landing?

2. Action:

   • If yes: Continue approach to landing.
   • If no: Immediately execute the missed approach procedure.

3. Missed Approach Execution:

   • Apply thrust/power, establish climb, retract gear/flaps as per procedure.
   • Follow the published missed approach routing for obstacle clearance and traffic separation.
   • Communicate with ATC as soon as practicable.

CRM (Crew Resource Management) practices require clear verbal confirmation (“Landing” or “Go-Around”) at DH to ensure both pilots are aware of and agree with the action.

Descending below DH without required visual references is a serious breach of regulations and jeopardizes safety.

Practical Examples

1. Standard CAT I ILS Approach

• DH: 200 ft AGL, RVR: 550 m.
• At 200 ft AGL, pilot acquires runway threshold and approach lights, confirms stabilized approach, and lands.

2. Low-Visibility CAT II ILS Approach

• DH: 100 ft AGL, RVR: 350 m.
• At 100 ft, pilot cannot see runway environment; initiates missed approach and notifies ATC.

3. Business Jet CAT IIIa Approach

• DH: 50 ft AGL, RVR: 200 m.
• Autoland engaged; at 50 ft, pilot acquires visual cues and continues to landing.

4. CAT IIIc (No DH) Autoland

• No DH; autoland in zero/zero visibility.
• Landing completed entirely by autopilot and certified airport systems.

Common Misunderstandings

• DH is not a recommendation; it is a legal and operational limit.
• Weather does not allow pilots to lower DH; only regulatory NOTAMs can increase minima, never decrease.
• DH applies only to approaches with vertical guidance and radio altimeter capability; DA and MDA apply in other contexts.
• Adherence to DH is tested in pilot training and proficiency checks.

Summary

Decision Height (DH) is a vital, regulated safety minimum in precision instrument approaches. It ensures pilots make standardized, timely decisions under IFR, preserving the highest levels of safety in challenging operating environments. Understanding DH, its operational context, and strict compliance is essential for all pilots and aviation professionals.

For further reading and authoritative references, consult ICAO Annex 6, FAA Instrument Procedures Handbook, and your national aviation authority’s regulations.